Soaps are said to be water-soluble sodium or potassium salts of fatty acids. Natural soaps are sodium or potassium salts of fatty acids, originally made by boiling lard or other animal fat together with potash (potassium hydroxide).
Saturated fatty acids such as stearic and palmitic etc. contain only single bonds in their molecule, while unsaturated fatty acids such as oleic, linoleic etc., contain one or more double bonds. Thus, soaps are usually a mixture of the sodium salts of the following acids:
- Stearic acid as sodium stearate (C17H35COONa) - saturated fatty acid; from vegetable oils like linseed oil, soyabean oil.
- Palmitic acid as sodium palmitate (C15H31COONa) - saturated fatty acid; Palm oil, animal fat
- Oleic acid as sodium oleate (C17H33COONa) - unsaturated fatty acid; Vegetable oils like linseed oil, soyabean oil.
- When soap is made from the sodium salts of the acids of cheap oils or fats, the resulting soap is hard. These soaps contain free alkalis and are mainly used as washing bars for laundry. When soap is prepared from the potassium salts of the acids of good grade oils and fat, it results in soft soap. These soaps do not contain free alkalis. They produce more lather and are used mainly as toilet soaps, shaving cream and shampoos.